NASA/ADS

The break-up of Rodinia, birth of Gondwana, true polar wander and the snowball Earth

Abstract

A major global plate reorganisation occurred between ̃750 and ̃550 Ma. Gondwana was assembled following the dispersal of Rodinia, a supercontinent centred on Laurentia in existence since ̃1050 Ma. The reorganisation began when tectonic elements, later composing East Gondwana, rotated piecemeal away from the Pacific margin of Laurentia. These elements swept across the ancestral Pacific (Mozambique) Ocean that lay between Laurentia and the combined African cratons of Congo and Kalahari, which were loosely joined after ̃820 Ma. Simultaneously, the Adamastor (Brasilide) Ocean closed by subduction bordering the West Gondwana cratons, drawing virtually all of Gondwana together by ̃550 Ma. The final assembly of Gondwana occurred contemporaneously with the separation of Laurentia from West Gondwana. It has been postulated that the imprint of Rodinia's long-lived existence on lower mantleconvection produced a prolate ellipsoidal geoid figure. This could give rise to episodic inertial interchange true polar wander (IITPW), meaning that the entire silicate shell of the Earth (above the core-mantle boundary) rolled through 90° with respect to the diurnal spin axis in ̃15 Ma (equivalent to an apparent polar wander velocity of ̃66 cm a -1. Although empirical arguments for IITPW of Cambrian age appear to be flawed, evidence for an ultra-fast ( > 40 cm a -1) meridional component of apparent polar wander for Laurentia between 564 and 550 Ma suggests that IITPW might have occurred at that time. The break-up of Rodinia increased the continental margin area and preferential organic C burial globally, which is reflected by high δ 13C values in seawater proxies. The consequent drawdown of CO 2 is implicated in a succession of runaway ice-albedo catastrophes between ̃750 and ̃570 Ma, during each of which the oceans completely froze over. Each "snowball" Earth event must have lasted for millions of years because their terminations depended on extreme CO 2 levels, built up by subaerial volcanic outgassing in the absence of sinks for C. A succession of ice-albedo catastrophes, each terminated under ultra-greenhouse conditions, must have imposed an intense environmental filter on the evolution of life. They may have triggered the radiation of Ediacaran fauna in the aftermath of the final snowball event. It is increasingly recognised that the Late Neoproterozoic was one of the most remarkable periods in Earth history, and it appears to exemplify the interplay of tectonics, the environment and biology in deep time.